In this program we will study three types of DNA synthesis in mutagen-sensitive strains of Drosophila melanogaster. A semiconservative DNA synthesis will be analyzed in mutant tissue culture cells with the following steps: 1. Mutants will be analyzed for defects in DNA synthesis with the technique of alkaline sucrose gradient centrifugation. 2. Chain elongation rates will be measured in defective mutants with isopycnic centrifugation and possibly with fiber autoradiography. 3. Cell synchronization techniques coupled with alkaline sucrose gradient centrifugation will be employed to characterize mutants that are defective in the process of joining completed replicons. 4. The sensitivity of the mutants to inhibitors of DNA synthesis will be characterized. B. Synthesis of DNA on a damaged template (bypass synthesis) will be studied in preblastoderm embryos of the mutants with the aid of the electronmicroscope. The influence of both monofunctional and bifunctional mutagens on the pattern of DNA synthesis will be analyzed. Data obtained by varying the mutagen and the mutant strain should make it possible to test some of the current models for bypass synthesis. C. Puff induction in polytene chromosomes of Drosophila will be analyzed after mutagen treatment in an attempt to identify loci involved in the induction of error-prone bypass synthesis.